Why was the East Coast earthquake felt so far from the epicenter?

Today's earthquake in Virginia was felt from South Carolina to Canada. We …

During today's magnitude 5.8 earthquake in Virginia, perceptible shaking was reported as far away as South Carolina and Maine. So why doesn't the same thing happen during California's much more frequent earthquakes along the San Andreas Fault? It comes down to a difference in crust. Density and temperature are primary controls on how far seismic waves can propagate through rock before dissipating

On the East Coast, the continental crust is older, colder, and denser. The coastline hasn't been tectonically active since Pangaea split apart, back when dinosaurs still roamed the Earth.

Contrast that with the West Coast, which is still tectonically active today, from the San Andreas Fault in Southern California to the subduction zone in the Pacific Northwest. West of the Rockies, the continent has been undergoing extension for the past 17 million years or so. That has stretched the crust, making it thinner and bringing hot mantle rock closer to the surface, which has warmed things up.

Because of these differences, shaking can be transmitted much farther (about three times the distance) through the colder, denser eastern crust.

95 Reader Comments

I think it has more to do with the rebounding of the northeast of the NA plate, when all that weight lets go at a weak point it will be felt up and down the plate because the whole plate will move a bit.

I think it has more to do with the rebounding of the northeast of the NA plate, when all that weight lets go at a weak point it will be felt up and down the plate because the whole plate will move a bit.

Mydrrn and the author of the article, could you please provide citations?

I was on a conference call with folks in our Virgina office this morning when they suddenly said their building is shaking. They're on the 18th floor and thought it was high winds; they're 3 hours from DC (I presume by driving).

Funky; I'm an hour from SF and the only time we were ever aware of a trembler was 15 years ago and even then it was a minor vibration of the floor (at Intel, huge building, 3rd floor, so perhaps it was more sensitive).

"Scott has a master's in hydrogeology from the University of Wisconsin- Madison. He's an avid collector of part-time jobs, but primarily works as an Earth science instructor at Madison College and as a hydrogeologist for the Wisconsin Department of Natural Resources"

Well... I'm no hydrogeologist... but I suspect that Scott knows more about the subject then I do.

I think the structural rigidity of the crust may have something to do with it as well. e.g. the west coast is carved up by faults that have moved a lot recently, as a result all of these faults can readily move miniscule amounts to absorb a shock wave.

I don't think Mydrryn is on target, the rebound (presumably referring to isostatic rebound following the removal of ice sheets) is a very different process and would not translate into improved seismic wave transmission.

citations: me (hey, I'm a geologist, albeit, not that kind of geologist)... yeah, I know, shame on me.

I guest it depends where you are too. There's reports of people who felt it in Montreal and called 911. However I'm one and half hour drive to the east of Montreal, and didn't felt a thing. I guest the trembler has been stopped by the Adirondack.

So if I rephrase in an way that easier to understand for most people, there's one big turtle under the east coast, that was pretty much just chillin' so when it got startled it jumped and everyone on top of that turtle felt it move.

But on the west coast, there's actually a lot of smaller turtles and they're already pretty much on the ball and just because one turtle spazzes out the other ones don't really mind and just think "hey that dude's spazzin' again" and so only the people above the spazzin' turtle feel it.

I was on the first floor of an office building in Baltimore, MD and we felt it pretty good here. Wasn't scary, but people on the third floor seemed to have felt it a lot more than I did. Windows and other things that were hanging shook a bunch, but nothing fell. Took about 5 seconds to figure out what was happening, another 5 for it to stop, then everyone came running down from upstairs.

So if I rephrase in an way that easier to understand for most people, there's one big turtle under the east coast, that was pretty much just chillin' so when it got startled it jumped and everyone on top of that turtle felt it move.

But on the west coast, there's actually a lot of smaller turtles and they're already pretty much on the ball and just because one turtle spazzes out the other ones don't really mind and just think "hey that dude's spazzin' again" and so only the people above the spazzin' turtle feel it.

I'm in Barrie, Ontario... pretty damn far away and I still felt it. Actually, it was weird... I was just shutting my eyes for a power nap and at first I wasn't sure if the swaying was real or part of my hazy brain.

So if I rephrase in an way that easier to understand for most people, there's one big turtle under the east coast, that was pretty much just chillin' so when it got startled it jumped and everyone on top of that turtle felt it move.

But on the west coast, there's actually a lot of smaller turtles and they're already pretty much on the ball and just because one turtle spazzes out the other ones don't really mind and just think "hey that dude's spazzin' again" and so only the people above the spazzin' turtle feel it.

Actually both coasts only have one turtle, but on the west coast, the plate is actually held up by four elephants standing on the turtle's back. These act as shock absorbers

I guest it depends where you are too. There's reports of people who felt it in Montreal and called 911. However I'm one and half hour drive to the east of Montreal, and didn't felt a thing. I guest the trembler has been stopped by the Adirondack.

I'm in Vermont, and neither the wife nor I felt a thing. Neither did my brother in law in Rhode Island, nor my in laws further north of us in Vermont.

So I don't know much about geology and was theorizing about a couple of things with some friends, all we can really do is apply our knowledge of mechanical waves in mediums(Sound waves and acoustics) to this and this article agrees with the reasoning we came up with for this, however there is one thing I haven't been able to understand yet and would be curious about.

Why exactly is there such diversity in the reported strengths of the earthquake? For instance, I was located a little over 20 miles form the epicenter, and honestly I managed to somehow miss the earthquake entirely, but those I was with did feel it and thought it was merely a train going by us. Near the epicenter there was obviously structural damage to the schools there, though knowing country schools in this area it wouldn't surprise me if that was cheap construction as well. Yet I am hearing reports of it shaking buildings to the point of them being evacuated in DC and Richmond which are both much farther away from what should have been a relatively minor earthquake.

My initial guess would have to be due to changes in the earth's density at various points along the paths, but would that by itself result in such a strong division in strengths of the earthquake?

There are two things that also make a difference, depth (as someone else mentioned) and peak ground acceleration.

It used to be that all I knew about earthquakes could be summed up with "higher Richter = stronger = stronger = bad". Then I moved to Christchurch. Since I've been here I've learned that a 7.1 can mess stuff up, but a 6.3 that is shallow and has higher peak ground acceleration can, well, the term used here is "munt". So a 6.3 munted the hell out of the city and killed a bunch of people. I've also learned, after about a billion aftershocks (three 4.0+'s this week) to no longer mock or look down on anyone's reaction to any quake.

Looking at the depth, coupled with Scott's explanation of the geological differences (and the turtles.. never trusted those things) I'm hardly surprised that people felt it all over the place.

I guest it depends where you are too. There's reports of people who felt it in Montreal and called 911. However I'm one and half hour drive to the east of Montreal, and didn't felt a thing. I guest the trembler has been stopped by the Adirondack.

I'm in Vermont, and neither the wife nor I felt a thing. Neither did my brother in law in Rhode Island, nor my in laws further north of us in Vermont.

I'm not sure what the qualifiers are, but we didn't feel a thing.

I was at work in Massachusetts...some people there felt it and some didn't -- it wasn't very strong. I only noticed it because my monitor was rocking back and forth a bit, and then because I started paying attention I realized I could feel my chair rocking. It felt like other distant quakes we have experienced in the northeast over the years (we do occasionally get them around here, just not very often), otherwise I just would have assumed it was due to the construction that has been going on outside the building, or maybe somebody moving something heavy on the floor above me.

So I don't know much about geology and was theorizing about a couple of things with some friends, all we can really do is apply our knowledge of mechanical waves in mediums(Sound waves and acoustics) to this and this article agrees with the reasoning we came up with for this, however there is one thing I haven't been able to understand yet and would be curious about.

Why exactly is there such diversity in the reported strengths of the earthquake? For instance, I was located a little over 20 miles form the epicenter, and honestly I managed to somehow miss the earthquake entirely, but those I was with did feel it and thought it was merely a train going by us. Near the epicenter there was obviously structural damage to the schools there, though knowing country schools in this area it wouldn't surprise me if that was cheap construction as well. Yet I am hearing reports of it shaking buildings to the point of them being evacuated in DC and Richmond which are both much farther away from what should have been a relatively minor earthquake.

My initial guess would have to be due to changes in the earth's density at various points along the paths, but would that by itself result in such a strong division in strengths of the earthquake?

Seablade

It could depend on the geologic conditions between you and the focus, but I don't know how significant that would be in this case. It's more likely related to the local sediment- certain conditions can amplify the shaking in that area. A classic example of this is the San Francisco bay muds.Also, there are differences between structures. That could be structural integrity, etc., or it could simply be size. (i.e. different buildings resonate with different frequencies of shaking)--OR--What if you sneezed and missed it?

There are two things that also make a difference, depth (as someone else mentioned) and peak ground acceleration.

It used to be that all I knew about earthquakes could be summed up with "higher Richter = stronger = stronger = bad". Then I moved to Christchurch. Since I've been here I've learned that a 7.1 can mess stuff up, but a 6.3 that is shallow and has higher peak ground acceleration can, well, the term used here is "munt". So a 6.3 munted the hell out of the city and killed a bunch of people. I've also learned, after about a billion aftershocks (three 4.0+'s this week) to no longer mock or look down on anyone's reaction to any quake.

Looking at the depth, coupled with Scott's explanation of the geological differences (and the turtles.. never trusted those things) I'm hardly surprised that people felt it all over the place.

If anyone's interested in aggregate eye witness level reports the USGS is accumulating them too. The main caveat is that while they carefully explained all the questions like "did your large appliances fall over" the one about how strong the shaking was (the value shown on that map) was entirely subjective with zero guidance. The problem is that excepting the occasional transplant, we east coasters don't have anything to calibrate the scale with.

Why exactly is there such diversity in the reported strengths of the earthquake? For instance, I was located a little over 20 miles form the epicenter, and honestly I managed to somehow miss the earthquake entirely, but those I was with did feel it and thought it was merely a train going by us. Near the epicenter there was obviously structural damage to the schools there, though knowing country schools in this area it wouldn't surprise me if that was cheap construction as well. Yet I am hearing reports of it shaking buildings to the point of them being evacuated in DC and Richmond which are both much farther away from what should have been a relatively minor earthquake.

Distance from the fault and the type of geology between you and the fault matters, but the biggest factor affecting this is local geology, i.e. what kind of ground you are standing on. If you are standing on a thick layer of relatively soft soil then you will generally feel stronger shaking than someone a short distance away standing on bedrock.

The type of building you are in matters too. Someone at the top of a tall, flexible building will feel the building moving in response to the ground (probably a less sharp or less abrupt motion, but longer-lasting) compared someone standing on the ground next to the building who will feel the ground vibration directly. This is why the "did you feel it" questionnaire that the USGS does asks you what kind of building you were in and what floor you were on.

So I don't know much about geology and was theorizing about a couple of things with some friends, all we can really do is apply our knowledge of mechanical waves in mediums(Sound waves and acoustics) to this and this article agrees with the reasoning we came up with for this, however there is one thing I haven't been able to understand yet and would be curious about.

Why exactly is there such diversity in the reported strengths of the earthquake? For instance, I was located a little over 20 miles form the epicenter, and honestly I managed to somehow miss the earthquake entirely, but those I was with did feel it and thought it was merely a train going by us. Near the epicenter there was obviously structural damage to the schools there, though knowing country schools in this area it wouldn't surprise me if that was cheap construction as well. Yet I am hearing reports of it shaking buildings to the point of them being evacuated in DC and Richmond which are both much farther away from what should have been a relatively minor earthquake.

My initial guess would have to be due to changes in the earth's density at various points along the paths, but would that by itself result in such a strong division in strengths of the earthquake?

Seablade

It could depend on the geologic conditions between you and the focus, but I don't know how significant that would be in this case. It's more likely related to the local sediment- certain conditions can amplify the shaking in that area. A classic example of this is the San Francisco bay muds.Also, there are differences between structures. That could be structural integrity, etc., or it could simply be size. (i.e. different buildings resonate with different frequencies of shaking)--OR--What if you sneezed and missed it?